1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus having an electron gun assembly of in-line type which emits three electron beams disposed in one line on one plane, and particularly, to a color cathode ray tube apparatus having an electron gun assembly of dynamic focus method by which good convergence is obtained over the entire area of a screen.
2. Description of the Related Art
In general, a color cathode ray tube apparatus is constructed to have a structure in which a fluorescent screen consisting of three color fluorescent material layers which respectively radiate in blue, green, and red colors, and the three electron beams emitted from an electron gun assembly is deflected by a deflection magnetic field generated by a deflecting apparatus, so that the above fluorescent screen is scanned in the horizontal and vertical directions, thereby displaying a color image. In this kind of color cathode ray tube apparatus, the color cathode ray tube apparatus typically uses three electron beams emitted from the electron gun assembly which consist of a center beam passing through a horizontal plane and a pair of side beams, which are disposed in one line on one plane.
Also, in general, the color cathode ray tube apparatus of this in-line type uses an electron gun assembly which has three cathodes disposed in one line in a horizontal direction, electron beam generating portions respectively disposed adjacent to the cathodes in the direction toward the fluorescent screen, and a plurality of electrodes having an integral structure and forming a main lens portion. The main lens portion has a function of static convergence, and due to this function of the main lens portion, each of three electron beams is focused so as to form a small beam spot on the fluorescent screen. Simultaneously, the pair of side beams are so shifted as to be close to the center beam, and are converged onto one point of the fluorescent screen.
Therefore, in an electron gun assembly of the color cathode ray tube apparatus having this kind of main lens portion, there is a problem that static convergence undesirably changes as a focusing voltage is adjusted.
As a technique for adjusting the focus voltage and solving the problem of changes in static convergence, Jpn. Pat. Appln. KOKAI Publication No. 1-42109 discloses means for performing a first orbit correction for deflecting a pair of side beams in a direction in which the beams extend to be close to a center beam near electron beam passing holes formed in the cathode side of focusing electrodes, and for performing a second orbit correction for deflecting the pair of side beams in the direction in which the beams extend to be close to the center beam, thereby complementarily influencing the first orbit correction in the cathode side of focusing electrodes and the second orbit correction in the main lens portion, by means of two-stage orbit corrections made to the pair of side beams when the focusing voltage is adjusted.
Meanwhile, a color cathode ray tube apparatus which has a large screen and displays a highly definite image of high quality is greatly required. As an electron gun assembly of this kind of color cathode ray tube apparatus, various new electron gun assemblies have been developed. One of the assemblies is, for example, an electron gun assembly of a resistor division method disclosed in Jpn. Pat. Appln. KOKAI Publication 2-223136. This electron gun assembly is constructed so as to have a structure in which an anode voltage is divided by a resistor provided in a tube and supplied to an electrode forming a main lens. Therefore, a highly definite image of high quality can be displayed and a high reliability is ensured against a tube discharge.
Further, in this electron gun assembly of resistor division method, developments have been made to an electron gun assembly of dynamic focus method which changes a focusing voltage in synchronization with a deflection of an electron beam. In case of this electron gun assembly of the dynamic focus method, when a peripheral portions of a fluorescent screen is scanned with an electron beam, the focusing voltage is higher by about 1000 V than when a central portion of the screen is scanned. When the focus voltage is thus high, convergence at a peripheral portion of the fluorescent screen is offset by about 1.0 mm.
If the technical means disclosed in the above-mentioned Jpn. Pat. Appln. KOKAI Publication No. 1-42109 is adopted to reduce convergence in a peripheral portion of the fluorescent screen, an offset in convergence is not substantially reduced.
In general, since a tolerance of convergence offset in the peripheral portion of a fluorescent screen is 0.3 mm or less, a convergence offset of the electron gun assembly of the above dynamic focus method exceeds the tolerance and greatly reduces image quality.
As has been stated above, the main lens portion of the electron gun assembly of a color cathode ray tube apparatus has a focusing function of focusing three electron beams emitted from an electron gun assembly and a static convergence function of focusing the three electron beams. The three electron beams are focused so as to form a small beam spot on a fluorescent screen, and simultaneously, a pair of side beams are deflected in a direction extending to be close to a center beam. Therefore, the electron gun assembly of the color cathode ray tube apparatus has a problem in that static convergence changes when the focus voltage is adjusted.
In particular, in an electron gun assembly of resistor division method which has been recently developed as an electron gun assembly of a color cathode ray tube apparatus which displays a highly definite image of high quality, a convergence offset in a peripheral portion of a fluorescent screen exceeds a tolerance and thereby greatly reduces image quality.